1. Field of the Invention
The invention relates to a method of imaging an object by means of magnetic resonance (MR), which object is arranged in a substantially uniform, steady magnetic field, said method including the following steps:
measuring MR signals from the object by means of MR imaging pulse sequences, PA1 filtering the measured MR signals so as to suppress shot noise, a corrected value which is related to the measured MR signals being assigned to a measuring point of a measured MR signal to be corrected, and PA1 reconstructing an image from the filtered MR signals. PA1 determining a combination of a value of a parameter of the measuring point of the measured MR signal to be corrected and values of the parameter of measuring points in a vicinity of the measuring point to be corrected, and PA1 assigning the corrected value to the measuring point to be corrected if the combination exceeds a predetermined reference. The advantage of such a method resides in the fact that the bandwidth of the signals to be measured need not be greater than the frequency content of the MR signals which is to be expected on the basis of the MR imaging pulse sequence used, so that less expensive receivers can be used for the MR signals to be measured. The invention is based on the idea that a statistical distribution of MR signals corresponding to a large part of the k space can be first-order approximated by a Gaussian distribution of white noise. As a result, a measuring point to be corrected has very likely been influenced by the shot noise if said combination exceeds the predetermined reference. In that case, for example a value zero is assigned to the measuring point to be corrected. Even though this step introduces an error in the value of the measured MR signal, it has been found that the effect of this error in the image is less disturbing than the effect exerted on the image by shot noise in the MR signal.
The invention also relates to an MR device for carrying out such a method.
2. Description of Related Art
A method of this kind is known from U.S. Pat. No. 5,525,906. In the context of the present patent application a k space is to be understood to mean a spatial frequency domain in which a path is followed during the measurement of the MR signals by application of gradients to the static magnetic field. The path in the k space is determined by the time integral of the gradients applied during the interval from the excitation of the nuclear spins until the instant in time at which the MR signal is measured. The measured values of the MR signals which correspond to the most important part of the path or paths produce the inverse Fourier transformed values of an image of the object. Furthermore, the term gradient is to be understood to mean temporary magnetic fields which are superposed on a steady magnetic field and cause a gradient in the steady magnetic field. MR imaging methods often utilize gradients in three respective orthogonal directions. Generally speaking, a gradient in the first direction is referred to as a read-out gradient, a gradient in the second direction as a phase encoding gradient, and a gradient in the third direction as a selection gradient. A measuring point is to be understood to mean a sampled value of the MR signal received at a sampling instant which corresponds to a position of the path in the k space.
The known method is used to suppress shot noise in the measured MR signals. In the context of the present patent application the term shot noise is to be understood to mean brief pulses which have a duration of, for example 1 microsecond or less, and a high amplitude relative to the measured MR signal. Such shot noise is caused, for example by electric discharges within the MR device or in the vicinity thereof, for example in the clothing of the staff attending the MR device. In the reconstructed MR image the shot noise present in the MR signal causes artefacts in the form of, for example a wavy pattern or a sum of wavy patterns which forms a fabric-like structure. According to the known method, a shot-noise affected value of the measuring point, which is to be corrected in relation to the measured MR signals, is replaced by the corrected value. To this end, according to the known method the energy content of a frequency band of the spectrum of the measured MR signals which is beyond the frequency band of the imaging information in the MR signal is determined. Subsequently, if the energy content exceeds a predetermined reference, the corrected value is assigned to the measuring point to be corrected. It is a drawback of this method that it is necessary to measure MR signals having a bandwidth which is larger than the bandwidth of the MR information which is to be expected on the basis of the MR imaging pulse sequence used.